A standard quarter wave (λ/4) Fresnel rhomb laterally displaces the output beam with respect to the input beam. This lateral displacement can cause problems when inserting the rhomb in an already aligned optical setup. This note describes the use of two λ/8 rhombs to construct a λ/4 rhomb. The output of the proposed rhomb is colinear with the input beam, and allows a much more flexible optical alignment.
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